Emission of two major types of pollutants, namely sulfur dioxide and nitrogen oxides, from industrial flue gas must be reduced in order to protect atmospheric environment. However, conventional processes for reducing emissions using a desulfurizing process unit and a denitrating process unit separately have problems such as low reliability and high constructional and operational costs, etc., due to their complex process structures and enormous resource consumption. Thus, conventional processes for removing SOx and NOx separately are severely restricted in terms of pollution alleviation and environmental protection. To prevent and control pollution caused by SOx and NOx and thus protect the atmospheric environment effectively, there has long been a need for an integrated, environmentally friendly technology for reducing emissions, and expectation of an environmentally friendly technical solution of simultaneous desulfurization and denitrification of flue gas.
Internationally, the typical schemes in technical field of simultaneous desulfurization and denitrification of flue gas are mainly as following:
DESONOX process, developed by Degussa Company, Germany, etc. (see FIG. 6), wherein ammonia solution (NH3) is sprayed into flue gas firstly, and then the flue gas enters a fixed bed reactor with double-layer catalysts, the first of which is a selective catalytic reduction (SCR) catalyst made of noble metals such as vanadium, etc., to catalyze the reduction of NOx by NH3 to N2 and H2O, and the second of which is an oxidative desulfurization catalyst of noble metals to catalyze the oxidation of SO2 to SO3 which is absorbed by water and then condensed to form sulfuric acid;
PHOSNOX process, developed by University of California, USA, wherein NO is oxidized to NO2 by O3 and O produced by yellow phosphorus, while yellow phosphorus is oxidized to P2O5 which forms phosphoric acid, and the NO2 oxidizes SO2 in an aqueous solution to SO42− and itself is reduced to hydroxylamine disulphonate, ammonium salt and other S—N compounds;
PPCP process, developed by Fukui University of Technology, Japan, wherein high-energy electron beams generated by an accelerator are used to dissociate gaseous molecules in flue gas into O, OH, HO2 and other active groups which oxidize SO2 and NOx to form ammonium sulfate and ammonium nitrate after ammonia is added; and so on.
Domestically in China, patented or patent pending technical solutions of simultaneous desulfurization and denitrification of flue gas include: application No. 200610012525.2 developed by North China Electric Power University and entitled “Liquid Phase Flue Gas Desulfurizing and Denitrifying Purification Method and Apparatus”, wherein an absorption solution of sodium chlorite with a combined additive, including calcium hypochlorite, hydrogen peroxide solution, disodium hydrogen phosphate, etc., is used to absorb sulfur dioxide and nitrogen oxides from flue gas in an ejection bubbling process; and patent No. 01105698.3 developed by East China University of Science and Technology and entitled “Process For Removing NOx from Waste Gas”, i.e., a complexation-catalysis process with cobalt (II) bis(ethylenediamine), wherein a catalyst including metal oxides such as NiO, CO3O4 loaded on Al2O3 is used to oxidize NO to NO2 in an alkaline solution; and so on.
The existing technical solutions of simultaneous desulfurization and denitrification confront the common problems. To achieve simultaneous desulfurization and denitrification, noble catalysts and/or absorbents need to be added in the process. The use of chemical materials of absorbents and catalysts, particularly the noble catalysts, results in excessive consumption of resources and energy, complication of process, low reliability of the whole processing device, high constructional and operational costs, and secondary pollution in some instances. Thus, it is difficult to apply these schemes widely.